Method for producing an ultra high strength coated or not coated steel sheet and obtained sheet

ABSTRACT

A method for producing a cold rolled steel sheet having a tensile strength≥1470 MPa and a total elongation TE≥19%, the method comprising the steps of annealing at an annealing temperature AT≥Ac3 a non-treated steel sheet whose chemical composition contains in weight %: 0.34%≤C≤0.40%, 1.50%≤Mn≤2.30%, 1.50≤Si≤2.40%, 0%&lt;Cr≤0.7%, 0%≤Mo≤0.3%, 0.01%≤Al≤0.07%, the remainder being Fe and unavoidable impurities, quenching the annealed steel sheet by cooling it to a quenching temperature QT&lt;Ms transformation point and between 150° C. and 250° C., and making a partitioning treatment by re-heating the quenched steel sheet to a partitioning temperature PT between 350° C. and 420° C. and maintaining the steel sheet at this temperature during a partitioning time Pt between 15 seconds and 250 seconds.

The present invention concerns the manufacture of coated or non-coated high strength steel sheet having improved tensile strength and improved total elongation and the sheets obtained by this method.

To manufacture various equipment such as parts of body structural members and body panels for automotive vehicles, it is now usual to use bare, electro-galvanized, galvanized or galvannealed sheets made of DP (dual phase) steels multi-phase, complex phase or martensitic steels.

For example, a high strength multi-phase may include a bainite-martensitic structure with/without some retained austenite and contains about 0.2% of C, about 2% of Mn, about 1.5% of Si which would result in yield strength of about 750 MPa, a tensile strength of about 980 MPa, a total elongation of about 10%. These sheets are produced on continuous annealing line by quenching from an annealing temperature higher than Ac3 transformation point, down to an overaging temperature above Ms Transformation point and maintaining the sheet at the temperature for a given time. Optionally, the sheet is galvanized or galvannealed.

To reduce the weight of the automotive parts in order to improve their fuel efficiency in view of the global environmental conservation it is desirable to have sheets having improved strength-ductility balance. But such sheets must also have a good formability.

In this respect, it was proposed to produce sheets made of steel using so called quenched and partitioned having improved mechanical properties and good formability. Coated or non-coated (bare) sheets having, a tensile strength TS of about 1470 MPa and a total elongation of at least 19%, are targeted. These properties are targeted at least when the sheet is not coated or galvanized.

When the sheet is galvannealed, a tensile strength TS of at least 1470 MPa and a total elongation of at least 15%, preferably at least 16% are targeted.

Therefore, the purpose of the present invention is to provide such sheet and a method to produce it.

For this purpose, the invention relates to a method for producing a cold rolled steel sheet having a tensile strength TS of at least 1470 MPa and a total elongation TE of at least 16%, the method comprising the successive steps of:

-   -   annealing at an annealing temperature AT a cold rolled steel         sheet made of steel whose chemical composition contains in         weight %:         0.34%≤C≤0.40%         1.50%≤Mn≤2.30%         1.50≤Si≤2.40%         0%<Cr≤0.7%         0%≤Mo≤0.3%         0.01%≤Al≤0.08%         and optionally 0%≤Nb≤0.05%,

the remainder being Fe and unavoidable impurities, the annealing temperature AT being equal or higher than the Ac3 transformation point of the steel, to obtain an annealed steel sheet,

-   -   quenching the annealed steel sheet by cooling it down to a         quenching temperature QT lower than the Ms transformation point         of the steel, typically between 150° C. and 250° C., to obtain a         quenched steel sheet, and,     -   making a partitioning treatment by reheating the quenched steel         sheet at a partitioning temperature PT between 350° C. and         450° C. and maintaining the steel sheet at this temperature         during a partitioning time Pt between 15 seconds and 250         seconds.

Preferably, during quenching, the annealed steel sheet is cooled down to said quenching temperature at a cooling rate fast enough to avoid ferrite formation upon cooling, in order to obtain a quenched sheet having a structure consisting of martensite and austenite.

Preferably, the annealing temperature AT is between 870° C. and 930° C.

According to an embodiment, the total elongation TE of the cold rolled steel sheet is of at least 19%, the composition of the steel is such that 0%<Cr≤0.5%, 0%<Mo≤0.3%, and the partitioning time is between 15 seconds and 150 seconds. Preferably, according to this embodiment, there is no addition of Nb.

Thus, according to this embodiment, the invention relates to a method for producing a cold rolled steel sheet having a tensile strength TS of at least 1470 MPa and a total elongation TE of at least 19%, the method comprising the successive steps of:

-   -   annealing at an annealing temperature AT a cold rolled steel         sheet made of steel whose chemical composition contains in         weight %:         0.34%≤C≤0.40%         1.50%≤Mn≤2.30%         1.50≤Si≤2.40%         0%<Cr≤0.5%         0%<Mo≤0.3%         0.01%≤Al≤0.08%

the remainder being Fe and unavoidable impurities, the annealing temperature AT being equal or higher than the Ac3 transformation point of the steel, to obtain an annealed steel sheet,

-   -   quenching the annealed steel sheet by cooling it down to a         quenching temperature QT lower than the Ms transformation point         of the steel, typically between 150° C. and 250° C., to obtain a         quenched steel sheet, and,     -   making a partitioning treatment by reheating the quenched steel         sheet at a partitioning temperature PT between 350° C. and         450° C. and maintaining the steel sheet at this temperature         during a partitioning time Pt between 15 seconds and 150         seconds.

In two embodiments, after partitioning the steel sheet is cooled to the room temperature in order to obtain a non-coated steel sheet:

In the first of these two embodiments, the composition of the steel is such that 0.36%≤C≤0.40%, Cr<0.05% and Mo<0.05%, the quenching temperature is between 190° C. and 210° C. and the partitioning time Pt is between 90 seconds and 110 seconds.

In the second of these two embodiments, the composition of the steel is such that 0.34%≤C≤0.37%, 0.35%≤Cr≤0.45% and 0.07%≤Mo≤0.20%, the quenching temperature is between 200° C. and 230° C. and the partitioning time Pt is between 25 seconds and 120 seconds.

Preferably, the bare cold rolled steel is afterwards electro-galvanized.

In another embodiment, after partitioning the steel sheet is galvanized then cooled to the room temperature in order to obtain a coated steel sheet, the composition of the steel is such that 0.34%≤C≤0.37%, 0.35%≤Cr≤0.45% and 0.07%≤Mo≤0.20%, the quenching temperature is between 200° C. and 230° C. and the partitioning time Pt is between 25 seconds and 55 seconds.

Thus, in a preferred embodiment, the composition of the steel is such that, 0.35%≤Cr≤0.45% and 0.07%≤Mo≤0.20%, and preferably such that 0.34%≤C≤0.37%.

With this preferred embodiment, if after partitioning the steel sheet is cooled to the room temperature in order to obtain a non-coated steel sheet, the quenching temperature is preferably between 200° C. and 230° C. and the partitioning time Pt is preferably between 15 seconds and 120 seconds.

Still with this preferred embodiment, if after partitioning the steel sheet is galvanized then cooled to the room temperature in order to obtain a coated steel sheet, the quenching temperature is preferably between 200° C. and 230° C. and the partitioning time Pt is preferably between 25 seconds and 55 seconds.

According to another embodiment, the composition of the steel is such that 0.46%≤Cr≤0.7% and/or 0.03%≤Nb≤0.05%, and preferably such that 0%≤Mo≤0.005%.

According to this embodiment, after partitioning the steel sheet is preferably coated then cooled to the room temperature in order to obtain a coated steel sheet.

According to this embodiment, the coating step is for example a galvanizing step. Preferably, the quenching temperature is between 200° C. and 230° C. and the partitioning time Pt is between 50 seconds and 250 seconds.

The coating step may be a galvannealing step with an alloying temperature GA between 470 and 520° C., preferably between 480° C. and 500° C., the sheet being maintained at the alloying temperature GA for a time comprised between 5 s and 15 s. Preferably, the quenching temperature is then between 200° C. and 230° C. and the partitioning time Pt between 40 s and 120 s.

The invention relates also to a coated or non-coated steel sheet made of steel whose chemical composition comprises in weight %: 0.34%≤C≤0.40% 1.50%≤Mn≤2.30% 1.50≤Si≤2.40% 0%<Cr≤0.7% 0%≤Mo≤0.3% 0.01%≤Al≤0.08% and optionally 0%≤Nb≤0.05%,

the remainder being Fe and unavoidable impurities, the structure comprising at least 60% of martensite and between 12% and 15% of residual austenite, the tensile strength is at least 1470 MPa and the total elongation being at least 16%.

In a particular embodiment, the steel is such that 0%<Cr≤0.5% and 0%<Mo≤0.3%.

The total elongation of the sheet is preferably at least 19%.

Thus, the invention relates in particular to a coated or non-coated steel sheet made of steel whose chemical composition comprises in weight %: 0.34%≤C≤0.40% 1.50%≤Mn≤2.30% 1.50≤Si≤2.40% 0%<Cr≤0.5% 0%<Mo≤0.3% 0.01%≤Al≤0.08%

the remainder being Fe and unavoidable impurities, the structure comprising at least 60% of martensite and between 12% and 15% of residual austenite, the tensile strength is at least 1470 MPa and the total elongation being at least 19%.

In a particular embodiment, the steel sheet is non-coated, the composition of the steel is such that 0<Cr<0.05% and 0<Mo<0.05%, and the yield strength is higher than 1150 MPa. Preferably, there is no addition of Nb.

In another embodiment, the steel sheet is non-coated, the composition of the steel is such that 0.35≤Cr≤0.45% and 0.07≤Mo≤0.20%, and the yield strength is higher than 880 MPa, the tensile strength is higher than 1520 MPa, and the total elongation is of at least 20%. Preferably, there is no addition of Nb.

In another embodiment, the steel sheet is galvanized, the composition of the steel is such that 0.35%≤Cr≤0.45% and 0.07%≤Mo≤0.20%, the tensile strength is higher than 1510 MPa and the total elongation is at least 20%. Preferably, there is no addition of Nb.

Thus, according to a preferred embodiment, the composition of the steel is such that 0.35%≤Cr≤0.45% and 0.07%≤Mo≤0.20%. If the sheet is not coated, the yield strength may be higher than 880 MPa, the tensile strength higher than 1520 MPa and the total elongation of at least 20%. If the sheet is galvanized, the tensile strength may be higher than 1510 MPa and the total elongation of at least 20%.

According to another preferred embodiment, the composition of the steel is such that 0.46%≤Cr≤0.7%, and/or 0.03%≤Nb≤0.05%. Preferably, the composition of the steel is such that 0%≤Mo≤0.005%.

Preferably, with this preferred embodiment, at least one face of the sheet is galvanized or galvannealed.

The invention will now be described in details but without introducing limitations.

According to the invention, the sheet is obtained by heat treating a hot or preferably a cold rolled non-treated steel sheet made of steel which chemical composition contains, in weight %:

-   -   0.34% to 0.40% of carbon to ensure a satisfactory strength and         improve the stability of the retained austenite. This is         necessary to obtain a sufficient elongation. If carbon content         is too high, the hot rolled sheet is too hard to cold roll and         the weldability is insufficient.     -   1.50% to 2.40% of silicon in order to stabilize the austenite,         to provide a solid solution strengthening and to delay the         formation of carbides during partitioning with appropriate         procedures to prevent the formation of silicon oxides at the         surface of the sheet which is detrimental to coatability.     -   1.50% to 2.30% of manganese to have a sufficient hardenability         in order to obtain a structure containing at least 60% of         martensite, a tensile strength of more than 1470 MPa and to         avoid having segregation issues which are detrimental for the         ductility.     -   0% to 0.3% of molybdenum and 0% to 0.7% of chromium to increase         the hardenability and to stabilize the retained austenite in         order to strongly reduce austenite decomposition during         partitioning. The absolute zero value is excluded due to         residual amounts. According to an embodiment, the composition         comprises from 0% to 0.5% of chromium. When the steel sheet is         non-coated, the molybdenum and the chromium can be eliminated         and their contents can remain less than 0.05% each. When the         steel sheet is coated by galvanizing, the molybdenum content is         preferably from 0.07% to 0.20% and the chromium content is         preferably from 0.35% to 0.45%. As an alternative, when the         sheet is coated, in particular by galvannealing, the molybdenum         content is preferably lower than 0.005%, and the chromium         content is preferably from 0.46% to 0.7%. A molybdenum content         lower than 0.005% corresponds to the presence of molybdenum only         as an impurity or a residual.     -   0.01% to 0.08% of aluminum which is usually added to liquid         steel for the purpose of deoxidation, preferably.

The remainder is iron and residual elements or unavoidable impurities resulting from the steelmaking. In this respect, Ni, Cu, V, Ti, B, S, P and N at least are considered as residual elements which are unavoidable impurities. Therefore, generally, their contents are less than 0.05% for Ni, 0.05 for Cu, 0.007% for V, 0.001% for B, 0.005% for S, 0.02% for P and 0.010% for N.

Addition of microalloy elements such as Nb from 0 to 0.05% and/or Ti from 0 to 0.1% could be utilized to obtain the desired microstructure and an optimal combination of product properties.

In particular, when the sheet is coated, Nb may be added in an amount up to 0.05%. According to an embodiment, Nb is preferably comprised between 0.03 and 0.05%. According to this embodiment, the sheet is preferably coated, by galvanizing or galvannealing. A Nb content of 0.03 to 0.05% allows obtaining satisfactory tensile strength and elongation, in particular a tensile strength of at least 1470 MPa and an elongation of at least 16%, when the sheet is coated by galvanizing or galvannealing.

Thus, when the sheet is coated, in particular by galvannealing, the composition may comprise Nb in an amount between 0.03% and 0.05%, Cr in an amount between 0.46% and 0.7%, and no addition of Mo.

The non-treated steel sheet is a cold rolled sheet prepared according to the methods known by those who are skilled in the art.

After rolling the sheets are pickled or cleaned then heat treated and optionally hot dip coated.

The heat treatment which is made preferably on a continuous annealing when the sheet is not coated and on a hot dip coating line when the steel sheet is coated, comprises the following successive steps:

-   -   annealing the cold rolled sheet at an annealing temperature AT         equal or higher than the Ac3 transformation point of the steel,         and preferably higher than Ac3+15° C., in order to obtain an         annealed steel sheet having a structure completely austenitic,         but less than 1000° C. in order not to coarsen too much the         austenitic grains. Generally, a temperature higher than 870° C.         is enough for the steel according to the invention and this         temperature does not need to be higher to 930° C. Then the steel         sheet is maintained at this temperature i.e. maintained between         AT−5° C. and AT+10° C., for a time sufficient to homogenize the         temperature in the steel. Preferably, this time is of more than         30 seconds but does not need to be more than 300 seconds. To be         heated to the annealing temperature, the cold rolled steel sheet         is, for example, first heated to a temperature of about 600° C.         at a speed typically below 20° C./s then heated again to a         temperature of about 800° C. at a speed typically below 10° C./s         and eventually heated to the annealing temperature at a heating         speed below 5° C./s. In this case, the sheet is maintained at         the annealing temperature for a duration between 40 and 150         seconds.     -   quenching of the annealed sheet by cooling down to a quenching         temperature QT lower than the Ms transformation point between         150° C. and 250° C. at a cooling rate fast enough to avoid         ferrite formation upon cooling and preferably of more than 35°         C./second, in order to obtain a quenched sheet having a         structure consisting of martensite and austenite, then the final         structure contains at least 60% of martensite and between 12%         and 15% of austenite. If the steel contains less than 0.05% of         molybdenum and less than 0.05% of chromium, the quenching         temperature is preferably between 190° C. and 210° C. When the         steel sheet has to be galvanized and when the chemical         composition of the steel is such that 0.34%≤C≤0.37%,         0.35%≤Cr≤0.45% and 0.07%≤Mo≤0.20%, then the quenching         temperature is preferably between 200° C. and 230° C. When the         composition of the steel is such that 0.46%≤Cr≤0.7% and         0%≤Mo≤0.005%, the quenching temperature is also preferably         between 200° C. and 230° C.     -   reheating the quenched sheet up to a partitioning temperature PT         between 350° C. and 450° C. The heating speed is preferably at         least 30° C./s.     -   maintaining the sheet at the partitioning temperature PT for a         partitioning time Pt between 15 sec and 250 sec, for example         between 15 sec and 150 sec. During the partitioning step, the         carbon is partitioned, i.e. diffuses from the martensite into         the austenite which is thus enriched.     -   Optionally, cooling the sheet down to the room temperature if no         coating is desired or heating the sheet to a coating         temperature, hot dip coating the sheet and cooling it down to         the room temperature if a coating is desired. The hot dip         coating is, for example, galvanizing, and the coating         temperature is about 460° C. as it is known in the art.

The heating to the coating temperature is made preferably at a heating speed of at least 30°/s and the coating takes between 2 and 10 s.

According to a particular embodiment, the hot dip coating is galvannealing. In this embodiment, the partitioning time is preferably comprised between 40 s and 120 s, for example higher than or equal to 50 s and/or lower than or equal to 100 s.

The sheet is heated from the partitioning temperature PT to the coating temperature, which is in this case an alloying temperature, and cooled down to room temperature after galvannealing.

The heating to the alloying temperature is made preferably at a heating speed of at least 20° C./s, preferably at least 30° C./s.

Preferably, the alloying temperature is lower than 520° C. and higher than 470° C. Still preferably, the alloying temperature is lower than or equal to 500° C. and/or higher than or equal to 480° C.

The sheet is maintained at the alloying temperature for a time which is for example comprised between 5 s and 20 s, preferably between 5 s and 15 s, for example between 8 s and 12 s. Indeed, maintaining the sheet at the alloying temperature for more than 20 s, leads to a reduction of the ductility, in particular to a decrease in the total elongation of the sheet.

Whether or not a coating is applied, the cooling speed to the room temperature is preferably between 3 and 20° C./s.

When the sheet is not coated and the steel contains preferably less than 0.05% of chromium and less than 0.05% of molybdenum, then the partitioning time is preferably between 90 sec and 110 sec. With such treatment it is possible to obtain sheets having a yield strength of more than 1150 MPa, a tensile strength of more than 1470 MPa and a total elongation of more than 19%.

When the sheet is not coated and the steel contains 0.35% and 0.45% of chromium and between 0.07% and 0.20% of molybdenum, then the partitioning time is preferably between 15 sec and 120 sec. With such treatment it is possible to obtain sheets having a yield strength of more than 880 MPa, a tensile strength of more than 1520 MPa and a total elongation of more than 20%.

When the sheet is coated, the composition and the treatment parameters are preferably adjusted according to the two following embodiments.

According to a first embodiment, when the sheet is coated, the steel contains preferably between 0.35% and 0.45% of chromium and between 0.07% and 0.20% of molybdenum and the partitioning time Pt is preferably between 25 seconds and 55 seconds. In these conditions it is even possible to obtain coated steel sheet having a tensile strength higher than 1510 MPa and a total elongation of at least 20%.

According to a second embodiment, when the sheet is coated, the steel may comprise between 0.46 and 0.7% of Cr, less than 0.005% of Mo and between 0.03 and 0.05% of Nb. With this composition, the partitioning time is preferably higher than 30 s, still preferably higher than or equal to 50 s.

When the sheet is coated by galvanizing, the partitioning time may be as high as 230 s.

When the sheet is coated by galvannealing, the partitioning time Pt is preferably between 40 seconds and 120 seconds, still preferably between 50 and 100 seconds. The alloying temperature is preferably comprised between 470° C. and 520° C., still preferably between 480° C. and 500° C.

The sheet is preferably maintained at the alloying temperature for less than 20 s, preferably less than 15 s, and more than 5 s. In these conditions it is possible to obtain a galvannealed steel sheet having a tensile strength higher than 1470 MPa, even higher than 1510 MPa, and a total elongation of at least 16%.

As examples and comparison, it was manufactured sheets made of steels whose compositions in weight and characteristic temperatures such as Ac3 and Ms are reported in table I.

The sheets were cold rolled, annealed, quenched, partitioned and cooled to the room temperature or, galvanized after partitioning before being cooled to the room temperature.

The mechanical properties were measured in the transverse direction relative to the direction of rolling. As it is well known in the art, the ductility level is slightly better in the direction of rolling than in the transverse direction for such high strength steel. Measured properties are Hole expansion ratio HER measured according to the standard ISO 16630:2009, the yield strength YS, the tensile stress TS, the uniform elongation UE and the total elongation TE.

The conditions of treatment and the mechanical properties are reported in Table II for the non coated sheets and in Table III for the coated sheets.

In these tables, AT is the annealing temperature, QT the quenching temperature, PT the partitioning temperature. In Table II, GI is the temperature of galvanizing.

TABLE I Ref C Mn Si Cr Mo Al Ac3 Ms steel % % % % % % ° C. ° C. S180 0.29 2.02 2.44 0.004 Residual 0.059 920 290 (<0.003) S181 0.39 2.03 1.95 0.003 Residual 0.058 860 240 (<0.003) S80 0.36 1.99 1.95 0.41 0.088 0.045 850 250 S81 0.38 1.98 1.93 0.34 0.14 1.047 860 270

TABLE II Ex- AT QT PT Pt HE YS TS UE TE ample Steel ° C. ° C. ° C. sec % MPa MPa % % 1 S180 920 240 400 10 — 982 1497 11.4 15.9 2 S180 920 240 400 100 17 1073 1354 13.9 19.9 3 S180 920 240 400 500 — 1082 1309 13.2 18.4 4 S181 900 200 400 10 — 1095 1583 12.5 13.8 5 S181 900 200 400 100 21 1238 1493 13.0 19.4 6 S181 900 200 400 500 — 1207 1417 13.1 17.7 7 S80 900 220 400 10 — 925 1518 6.6 6.8 8 S80 900 220 400 30 — 929 1438 8.9 8.9 9 S80 900 220 400 50 — 897 1462 13.5 18.5 10 S80 900 220 400 100 — 948 1447 15.7 19.6 11 S81 900 240 400 10 — 867 1623 8.1 9.3 12 S81 900 240 400 30 — 878 1584 11.4 11.8 13 S81 900 240 400 50 — 833 1520 10.8 12.2 14 S81 900 240 400 100 — 840 1495 15.9 17.3

TABLE III Ex- AT QT PT GI Pt HE YS TS UE TE ample Steel ° C. ° C. ° C. ° C. sec % MPa MPa % % 15 S180 920 240 400 460 100 24 1127 1310 13.7 20.7 16 S181 900 200 400 460 10 — 933.4 1348 14.0 18.0 17 S181 900 200 400 460 30 — 1170 1425 13.8 20.1 18 S181 900 180 400 460 100 — 1353 1507 8.0 14.1 19 S181 900 200 400 460 100 19 1202 1399 13.0 20.2 20 S181 900 220 400 460 100 — 936 1280 14.3 18.0 21 S181 900 200 420 460 10 — 906 1346 11.2 10.6 22 S181 900 200 420 460 30 — 841 1298 14.7 19.3 23 S181 900 200 420 460 100 — 900 1322 14.5 19.1 24 S181 900 200 360 460 10 — 910 1357 14.5 19.0 25 S181 900 200 360 460 30 — 992 1356 14.0 18.9 26 S80 900 220 400 460 10 — 756 1576 10.5 11.1 27 S80 900 220 400 460 30 — 836 1543 18.3 20.3 28 S80 900 220 400 460 50 — 906 1534 18.6 21.6 29 S80 900 220 400 460 100 — 941 1394 8.1 8.58 30 S81 900 240 400 460 10 — 925 1658 9.4 9.39 31 S81 900 240 400 460 30 — 929 1603 15.1 20.5 32 S81 900 240 400 460 50 — 897 1554 16.1 21.1 33 S81 900 240 400 460 100 — 948 1542 18.1 21.4

The examples 1 to 14 show that it is only with the steel S181, which contains neither chromium nor molybdenum, and steel S80, which contains both chromium and molybdenum, that it is possible to reach the desired properties i.e. TS≥1470 MPa and TE≥19%. In alloy S181, the desired properties are achieved for a quenching temperature QT of 200° C. and a partitioning time of 100 seconds. In this case, the yield strength is higher than 1150 MPa. In alloy S80, which contains chromium and molybdenum, the desired properties are achieved for a quenching temperature QT of 220° C. and a partitioning time between 30 to 100 seconds (examples 7 to 10). In this case, the tensile strength is higher than 1520 MPa and the total elongation is more than 20%. Moreover, it is worth mentioning that all the examples containing Cr and Mo (7 to 14) have yield strengths significantly lower than the examples 1 to 6, concerning a steel without Cr and Mo.

The examples 15 to 33 show that only the examples corresponding to steels containing Cr and Mo are able to reach the desired properties when the sheets are galvanized (examples 27 and 28). For the steel S80, the quenching temperature has to be of 220° C. and a partitioning of 10 seconds is too short while a partitioning time of 100 seconds is too long. When the steel does not contain Cr and does not contain Mo, the tensile strength always remains lower than 1470 MPa.

Other sheets made of an alloy having the composition shown in Table IV were cold rolled, annealed, quenched, partitioned, galvanized or galvannealed and cooled to the room temperature.

TABLE IV C Mn Si Cr Mo Al Ac3 Ms % % % % % % Nb ° C. ° C. 0.38 1.98 1.93 0.51 0.003 0.048 0.039 825 290

The mechanical properties of the sheets were measured in the transverse direction relative to the direction of rolling. As it is well known in the art, the ductility level is slightly better in the direction of rolling than in the transverse direction for such high strength steel. Measured properties are the Hole Expansion Ratio HER measured according to the standard ISO 16630:2009, the yield strength YS, the tensile strength TS, the uniform elongation UE and the total elongation TE.

The conditions of treatment and the mechanical properties of the galvanized sheets are reported in Table V.

In this table, GI is the temperature of galvanizing.

TABLE V AT QT PT Pt GI HE YS TS UE TE Example ° C. ° C. ° C. sec ° C. % MPa MPa % % 34 900 205 400 30 460 — 1032 1624 14 15.7 35 900 205 400 50 460 — 1102 1606 16.1 19.8 36 900 205 400 150 460 — 1139 1594 15.3 20.9 37 900 205 400 230 460 — 1179 1606 15.2 19.2

Examples 35 to 37 show that with a steel comprising higher amounts of chromium and niobium and a lower amount of molybdenum, the desired properties, i.e. TS≥1470 MPa and TE≥19%, can be reached with a partitioning time of more than 30 s, in particular of at least 50 s.

The conditions of treatment and the mechanical properties of the galvannealed sheets are reported in Table VI.

In this table, TGA is the alloying temperature and t_(GA) is the holding time at this alloying temperature TGA.

AT QT PT Pt TGA t_(GA) HE YS TS UE TE Example ° C. ° C. ° C. sec ° C. sec % MPa MPa % % 38 900 205 400 30 500 20 — 850 1589 9.8 12.8 39 900 205 400 50 500 20 — 858 1563 12.1 12 40 900 205 400 100 500 20 — 881 1534 13.4 15.7 41 900 205 400 50 500 10 — 1062 1548 14.7 16.5 42 900 205 400 100 500 10 — 990 1561 14.3 16.5 43 900 205 400 150 500 10 — 998 1581 12.7 14.3 44 900 205 400 50 480 10 — 1035 1603 14.4 17.9

Examples 38-44 show that a partitioning time Pt between 40 seconds and 120 seconds, in particular between 50 and 100 seconds, allow obtaining a galvannealed steel sheet having a tensile strength higher than 1510 MPa and a total elongation of at least 16%.

In particular, example 44 show that an alloying temperature of 480° C. and a holding time at the alloying temperature of 10 s even allow obtaining a tensile strength of more than 1510 MPa and a total elongation of more than 16%, even more than 17%. 

The invention claimed is:
 1. A method for producing a coated steel sheet having a tensile strength TS of at least 1470 MPa and a total elongation TE of at least 16%, the method comprising the successive steps of: annealing at an annealing temperature AT a cold rolled steel sheet made of a steel having a chemical composition containing in weight %: 0.34%≤C≤0.40%; 1.50%≤Mn≤2.30%; 1.50≤Si≤2.40%; 0.35%≤Cr≤0.45%; 0.07%≤Mo≤0.20%; 0.01%≤Al≤0.08%; and 0%≤Nb≤0.05%; a remainder being Fe and unavoidable impurities, the annealing temperature AT being higher than an Ac3 transformation point of the steel, to obtain an annealed steel sheet; quenching the annealed steel sheet by cooling the steel sheet down to a quenching temperature QT lower than an Ms transformation point of the steel and comprised between 200° C. and 230° C., to obtain a quenched steel sheet; performing a partitioning treatment by reheating the quenched steel sheet at a partitioning temperature PT between 350° C. and 450° C. and maintaining the quenched steel sheet at the partitioning temperature PT during a partitioning time Pt between 25 seconds and 55 seconds; galvanizing the steel sheet, the galvanizing comprising heating the steel sheet from the partitioning temperature PT to a galvanizing temperature at a heating rate of at least 30° C./s; then cooling the steel sheet to room temperature in order to obtain the coated steel sheet.
 2. The method according to claim 1, wherein during quenching, the annealed steel sheet is cooled down to said quenching temperature at a cooling rate fast enough to avoid ferrite formation upon cooling, in order to obtain a structure of the quenched steel sheet consisting of martensite and austenite.
 3. The method according to claim 1, wherein the annealing temperature AT is between 870° C. and 930° C.
 4. The method according to claim 1, wherein the chemical composition of the steel includes 0.34%≤C≤0.37%.
 5. The method according to claim 1, wherein the annealing at the annealing temperature AT comprises heating the cold-rolled steel sheet to a temperature of 600° C. at a first heating speed below 20° C./s, then heating the cold-rolled steel sheet to a temperature of 800° C. at a second heating speed below 10° C./s, and heating the cold-rolled steel sheet to the annealing temperature AT at a heating speed below 5° C./s.
 6. The method according to claim 1, wherein the quenched steel sheet is reheated to the partitioning temperature PT at a heating rate of at least 30° C./s.
 7. The method according to claim 1, wherein the steel sheet is cooled to the room temperature at a cooling rate comprised between 3 and 20° C./s.
 8. A method for producing a coated steel sheet having a tensile strength TS of at least 1470 MPa and a total elongation TE of at least 16%, the method comprising the successive steps of: annealing at an annealing temperature AT a cold rolled steel sheet made of a steel having a chemical composition containing in weight %: 0.34%≤C≤0.40%; 1.50%≤Mn≤2.30%; 1.50≤Si≤2.40%; 0.35%≤Cr≤0.45%; 0.07%≤Mo≤0.20%; 0.01%≤Al≤0.08%; and 0%≤Nb≤0.05%; a remainder being Fe and unavoidable impurities, the annealing temperature AT being higher than an Ac3 transformation point of the steel, to obtain an annealed steel sheet; quenching the annealed steel sheet by cooling the steel sheet down to a quenching temperature QT lower than an Ms transformation point of the steel and comprised between 200° C. and 230° C., to obtain a quenched steel sheet; performing a partitioning treatment by reheating the quenched steel sheet at a partitioning temperature PT between 350° C. and 450° C. and maintaining the quenched steel sheet at the partitioning temperature PT during a partitioning time Pt between 25 seconds and 55 seconds; galvanizing the steel sheet at a galvanizing temperature within a time comprised between 2 s and 10 s; then heating the steel sheet from the galvanizing temperature to an alloying temperature GA between 470° C. and 520° C. at a heating speed of at least 20° C./s, maintaining the steel sheet at the alloying temperature GA for a time between 5 s and 15 s and cooling the steel sheet to room temperature.
 9. The method according to claim 8, wherein during quenching, the annealed steel sheet is cooled down to said quenching temperature at a cooling rate fast enough to avoid ferrite formation upon cooling, in order to obtain a structure of the quenched steel sheet consisting of martensite and austenite.
 10. The method according to claim 8, wherein the annealing temperature AT is between 870° C. and 930° C.
 11. The method according to claim 8, wherein the chemical composition of the steel includes 0.34%≤C≤0.37%.
 12. The method according to claim 8, wherein the annealing at the annealing temperature AT comprises heating the cold-rolled steel sheet to a temperature of 600° C. at a first heating speed below 20° C./s, then heating the cold-rolled steel sheet to a temperature of 800° C. at a second heating speed below 10° C./s, and heating the cold-rolled steel sheet to the annealing temperature AT at a heating speed below 5° C./s.
 13. The method according to claim 12, wherein the first heating speed is comprised between 20° C./s and 10° C./s, the second heating speed is distinct from the first heating and comprised 10° C./s and 5° C./s, and the heating speed from 800° C. to the annealing temperature AT is distinct from the second heating speed.
 14. The method according to claim 8, wherein the quenched steel sheet is reheated to the partitioning temperature PT at a heating rate of at least 30° C./s.
 15. The method according to claim 8, wherein the galvanizing comprises heating the steel sheet from the partitioning temperature to the galvanizing temperature at a heating rate of at least 30° C./s.
 16. The method according to claim 8, wherein the steel sheet is cooled to the room temperature at a cooling rate comprised between 3 and 20° C./s.
 17. The method according to claim 8, wherein the steel sheet is heated from the galvanizing temperature to the alloying temperature GA at a heating speed of at least 30° C./s. 